Normal human somatic cells, such as fibroblasts, endothelial cells, and epithelial cells, are characterized by a finite capacity to divide in vitro, after which they cease proliferating even in the presence of adequate growth factors. Typically, such cells are limited to about 50-100 in vitro doublings, after which they cease to divide. This cessation of replication in vitro is generally referred to as cellular senescence. See e.g., Goldstein, 1990; Hayflick & Moorehead, 1961; Hayflick, 1965; Ohno, 1979; Ham & McKeehan, 1979. Furthermore, the replicative life span of cells grown in vitro is often inversely proportional to the in vivo age of the donor (Martin et al., 1970; Goldstein et al., 1969; Schneider & Mitsui, 1976).
Some cells are able to escape cellular senescence, thereby retaining or perhaps acquiring unlimited replicative capacity in vitro (Shay et al., 1991). Such cells are generally referred to as “immortalized” to reflect the ability to remain replicatively competent over hundreds or even thousands of passages in culture. The immortalization process appears to involve one or more mutagenic events or the aid of transforming viral oncoproteins, and even then the frequency of immortalization is only about 10−6 to 10−7 in human cells (Shay & Wright, 1989). Thus, with the exception of germ cells and perhaps certain stem cells, mammalian somatic cells are mortal and can only be maintained for relatively short periods in culture.
A variety of hypotheses have been proposed over the years to explain the causes of cellular senescence, and it is clear that senescence and can result from several independent stimuli such as cancer-causing mutations (oncogene activation), oxidative stress, artifacts of cell culture, and telomere shortening. It should be noted, however, that no consensus has been reached concerning the cause of cellular senescence in the whole organism as opposed to in vitro, nor is the role of cellular senescence in organismal aging clearly established.
One theory that relates in senescence and aging is referred to as the free radical theory. The free radical theory of aging suggests that free radicals damage DNA and other macromolecules resulting in critical losses of cell and/or tissue functions (Harman, 1956; Harman, 1961). The somatic mutation theories propose that the progressive accumulation of genetic damage to somatic cells by radiation and other environmental insults impairs cell function and that somatic cells because somatic cells lack the ability to repair the damage through recombination, they fail to proliferate indefinitely (Burnet, 1974; Hayflick, 1992).
Shay et al. describe a two-stage model for human cell mortality to explain the ability of Simian Virus 40 (SV40) T-antigen to immortalize human cells. This model likely explains a subset of the forms of cellular senescence, and is not mutually exclusive with other theories as senescence such as the free radical theory (that is, certain forms of DNA damage may alter telomere structure to induce senescence). See Shay et al., 1991 and Shay et al., 1992. The mortality stage 1 mechanism (M1) is the target of certain tumor virus proteins, and an independent mortality stage 2 mechanism (M2) produces crisis and prevents these tumor viruses from directly immortalizing human cells. These authors utilized T-antigen driven by a mouse mammary tumor virus promoter to cause reversible immortalization of cells. The Simian Virus 40 T-antigen is said to extend the replicative life span of human fibroblast by an additional 40-60%. The authors have established that the M1 mechanism is overcome by T-antigen binding to various cellular proteins (p53 and RB). The M2 mechanism then causes cessation of proliferation, even though the M1 mechanism is blocked. Immortality is achieved only when the M2 mortality mechanism is also disrupted. It is now known that both M1 and M2 result from alterations of telomere structure in some models of senescence in human cells.
Numerous publications have suggested that aspects of telomere length and replication are associated with cellular senescence. It has also been proposed that the finite replicative capacity of cells might reflect the work of a “clock” linked to DNA synthesis in the telomere (end part) of the chromosomes. In 1973, Olovnikov described the theory of marginotomy to explain the limitations of cell doubling potential in somatic cells. He stated that an:                informative oligonucleotide, built into DNA after a telogene and controlling synthesis of a repressor of differentiation, might serve as a means of counting mitosis performed in the course of morphogenesis. Marginotomic elimination of such an oligonucleotide would present an appropriate signal for the beginning of further differentiation. Lengthening of the telogene would increase the number of possible mitoses in differentiation.Olovnikov, 1973. Harley et al. stated that the amount and length of telemetric DNA in human fibroblasts decreases as a function of serial passage during aging in vitro, and possibly in vivo, but do not know whether this loss of DNA has a causal role in senescence (Harley et al., 1990). They point out, however, that:        Tumour cells are also characterized by shortened telomeres and increased frequency of aneuploidy, including telomeric associations. If loss of telomeric DNA ultimately causes cell-cycle arrest in normal cells, the final steps in this process may be blocked in immortalized cells. Whereas normal cells with relatively long telomeres and a senescent phenotype may contain little or no telomerase activity, tumour cells with short telomeres may have significant telomerase activity. Telomerase may therefore be an effective target for anti-tumour drugs.        
Thus, it appears that cellular senescence might correlate with telomere shortening over the life span of the mammal. In fact, Harley discusses observations allegedly showing that telomeres of human somatic cells act as a mitotic clock shortening with age both in vitro and in vivo in a replication dependent manner. He states:                Telomerase activation may be a late, obligate event in immortalization since many transformed cells and tumour tissues have critically short telomeres. Thus, telomere length and telomerase activity appear to be markers of the replicative history and proliferative potential of cells; the intriguing possibility remains that telomere loss is a genetic time bomb and hence causally involved in cell senescence and immortalization . . .        Despite apparently stable telomere length in various tumour tissues or transformed cell lines, this length was usually found to be shorter than those of the tissue of origin. These data suggest that telomerase becomes activated as a late event in cell transformation, and that cells could be viable (albeit genetically unstable) with short telomeres stably maintained by telomerase. If telomerase was constitutively present in a small fraction of normal cells, and these were the ones which survived crisis or became transformed, we would expect to find a greater frequency of transformed cells with long telomeres.Harley, 1991 (Citations Omitted).        
In the non-malignant cell, it appears that several signals such as telomere shortening and oxidative stress can provoke senescence. Nonetheless, while cellular aging and telomere shortening might generally correlate in vitro and/or in vivo, the observation that many tumor cells have critically shortened telomeres makes indicates that telomere length is not an absolute predictor of the molecular or physiologic age of a cell.
Thus, there is a long-felt and continuing need in the art for new methods for determining the molecular or physiologic age of a cell or tissue.